The present invention is concerned with a liquid dispensing device. More specifically, the present invention is concerned with a small volume droplet dispensing device, in particular for volumes in the order of 50 nanoliters to 100 microliters.
Within certain fields, such as protein crystallography, it is desirable to be able to dispense small volume samples, or droplets, of a given liquid. Such samples are dispensed at high speed into a well plate for subsequent analysis. A known droplet dispensing apparatus comprises a syringe defining a cylinder with a piston disposed therein. The liquid to be dispensed is stored within the cylinder and the piston is advanced in order to dispense controlled volumes of droplets.
Contact based dispensing systems are known in the art, in which a droplet is pushed from the syringe and put in contact with an external surface. The surface tension of the liquid allows the droplet to be dispensed by contact with the surface.
EP1344565 discloses a droplet dispensing apparatus in which a solenoid is used to “tap” the end of a piston rod and thereby advance a piston within a cylinder to dispense droplets of liquid. EP1344565 discloses an output member in the form of a drive rod attached to the solenoid which is spaced from a piston rod head attached to the piston rod. The solenoid accelerates the drive rod, which hits the head once the drive rod has built up speed, transferring momentum to the piston rod and liquid. The piston rod is connected to a striker, which striker is arranged to abut an adjustable anvil to arrest the movement of the piston rod. The mechanism facilitates droplet dispensing by suddenly stopping the piston and allowing a drop of liquid to be ejected under its own momentum.
The velocity of the droplet means it is “fired” from the syringe and does not require contact with a surface to be dispensed.
There are various problems with this approach. The collision due to this type of tapping motion causes shockwaves in the liquid which can cause dispensing problems. These problems include the dispensing of satellite droplets, as well as ingress of air into the syringe, both of which are undesirable. The shockwaves are caused by the collision between the moving drive rod and the stationary piston rod head (i.e. as a result of the gap between the drive rod and the piston rod head).
It is desirable to use removable/replaceable syringes for this type of apparatus. This prevents contamination, and allows different syringes to be used as the user requires.
A problem with the prior art is that when attaching a new syringe, the piston rod of the dispensing apparatus needs to be manually attached to the syringe piston. This can be labour intensive and time consuming.
Similarly, at the end of the dispensing operation, the syringes need to be separate from the piston rods in order to replace or clean them for re-use. Such removal can be time consuming.
A further problem with dispensing of liquids in this manner is splashing. Typically, a well-plate is provided in which several wells are formed to receive the dispensed liquids. Should any of the liquids splash from the wells it may enter adjacent wells or simply rest on top of the well plate. Both of these outcomes are undesirable.